The present invention relates to the folding of sheet-material blanks and more particularly to a method and an apparatus for effecting such folding.
Sheet-material blanks must frequently be folded, for instance to form a signature in a book, brochure or the like. Such folding is ususally carried out by means of a longitudinal or transverse folding device associated with so-called rotary folders of the type used in various rotary printing machines which use sheet material. As a rule, these folding devices use a conically or analogously convergent guide in which the sheet material blank is made to advance whereby it becomes progressively folded. Another approach known from the art is to use transverse folding cylinders. In some instances, where the fold must be carried out with particular accuracy, so-called knife-folders are employed. These "knives" are folding elements which are guided to perform a movement during which they engage the respective sheet material blank and fold it. It is known to guide the knife in a straight-line guide or to articulate it to a swingable arm.
The problem with these latter types of folders is the drive for the movement which is to be performed by the knife, particularly if the knife is required to form a fold on a large-format sheet-material blank. This means that the knife must pass through a relatively significant stroke, and of course in modern equipment the high-speed operation of the machines necessitates that the number of strokes be correspondingly great. This means that a cam control of the knife movement, the inherently ideal way of controlling the movement, is no longer possible. It has already been attempted to effect the control by means of electromagnetic units. These last two types of drive arrangements have the disadvantage that the number of strokes which is required in modern machines per unit of time, cannot be achieved. Moreover, the drives accelerate the knife or deflecting element in such a manner that it engages the respective sheet-material blank in shock-like manner, a disadvantage in terms of the accuracy of folding which can be obtained.
Some improvement was obtained in this field by using straight-line articulated crank linkages. Such crank linkages are driven and the output member of the linkage is associated with the knife which it then moves in a straight line. The difficulty with these arrangements is that the speed of movement of the last element, that is here the knife, decreases significantly long before the knife reaches the end of its working stroke. This means that that the sheet-material blank engaged by the knife receives only a relatively brief but very strong impulse deflecting it out of the path in which it travels, but is not positively guided subsequently because of the progressive retardation in the speed of advancement of the knife. Thus, the deflected portions of the sheet-material blank will slide more or less freely under the impulse initially imparted to them by the knife, in the direction in which they have been deflected by the knife, before they are engaged by the folding rollers which then complete the folding operation. In many instances, however, the sheet-material blanks have already previously been folded by means of conical folding guides or other devices, so that they are of V-shaped configuration, that is they are already bi-folded in one direction. This means that at the opposite sides of the sheet-material blank, that is at the opposite lateral edges thereof, different stiffness and friction coefficients exist. This, in turn, means that as soon as the blank is no longer positively guided by contact with the knife, it will tend to shift its configuration and position and depending upon the mass and the speed of the initial impulse imparted to it, it will reach the folding rollers at some angle of inclination. This, as is well known to thse conversant with the art, results in various types of difficulties.